Bash Control Structures If statement
Example
if [[ $1 -eq 1 ]]; then
echo "1 was passed in the first parameter"
elif [[ $1 -gt 2 ]]; then
echo "2 was not passed in the first parameter"
else
echo "The first parameter was not 1 and is not more than 2."
fi
The closing fi is necessary, but the elif and/or the else clauses can be omitted.
The semicolons before then are standard syntax for combining two commands on a single line; they can be omitted only if then is moved to the next line.
It's important to understand that the brackets [[ are not part of the syntax, but are treated as a command; it is the exit code from this command that is being tested. Therefore, you must always include spaces around the brackets.
This also means that the result of any command can be tested. If the exit code from the command is a zero, the statement is considered true.
if grep "foo" bar.txt; then
echo "foo was found"
else
echo "foo was not found"
fi
Mathematical expressions, when placed inside double parentheses, also return 0 or 1 in the same way, and can also be tested:
if (( $1 + 5 > 91 )); then
echo "$1 is greater than 86"
fi
You may also come across if statements with single brackets. These are defined in the POSIX standard and are guaranteed to work in all POSIX-compliant shells including Bash. The syntax is very similar to that in Bash:
if [ "$1" -eq 1 ]; then
echo "1 was passed in the first parameter"
elif [ "$1" -gt 2 ]; then
echo "2 was not passed in the first parameter"
else
echo "The first parameter was not 1 and is not more than 2."
fi
